1. Field of the Invention
The present invention relates to a method and apparatus for waste water treatment which can effectively treat by biochemical reaction fluorine, organic substances, such as surfactants, and hydrogen peroxide (H.sub.2 O.sub.2) that are present in waste water.
2. Description of the Prior Art
Recently, nonuse of fluorocarbon has been strongly called for. In view of this need of the times, industrial establishments, such as semiconductor and liquid crystal factories in particular, are beginning to use different kinds of surfactants, alcohol-based organic solvents, fluorine-based solvents, and the like as substitutes for cleaning-purpose fluorocarbon. Also, from the standpoint of process refinement, such surfactants and organic solvents are actually used.
Especially, cleaning by a combination of ultrapure water and a surfactant is expected to be a vital substitute cleaning method oriented toward total nonuse of fluorocarbon, when considered in terms of detergency and possible effects on parts. As such, operations in semiconductor and liquid crystal factories have a tendency to increase effluent containing fluorine and surfactants used in the process of wafer fabrication.
Generally, referring first to fluorine, in the case of semiconductor factories, fluorine-containing effluent often has a fluorine concentration on the order of 30 to 300 ppm. An influent containing fluorine of such concentration is subjected to waste water treatment in such a way that a chemical, such as slaked lime, polyaluminum chloride, or polymer coagulant, is added to the waste water to form flocs of a hardly soluble calcium fluoride or aluminum fluoride. The flocs of calcium fluoride or aluminum fluoride are then coagulated by a coagulant into larger flocs, and the flocs thus formed are then precipitated for separation.
Referring next to surfactants, it is to be noted that microfabrication techniques will further progress in up-to-date semiconductor and liquid crystal factories, and that there will be a difficulty in cleaning minute parts with ultrapure water because of its surface tension. For this reason, in a cleaning method which is now widely in practice such that a surfactant and/or an organic substance, such as alcohol, is added to ultrapure water, whereby the surface tension of water is reduced to enable cleaning microfine portions. However, such materials as surfactants and the like can hardly be microbially degraded because of their molecular and structural formulas, foaming characteristics, and germicidal effect. Nevertheless, it is anticipated that microfabrication will progress much further in semiconductor and liquid crystal factories and, therefore, surfactants and the like contained in fluorine-containing effluent will tend to increase in proportion to the progress of such microfabrication. Therefore, in conjunction with the treatment of above said fluorine content, it is also necessary to treat waste water containing organic substances, mainly surfactants, in a reasonable and economical manner.
Also it is anticipated that fluorine will continue to be contained in effluent from semiconductor and liquid crystal factories because fluorine-containing ultrapure water has good cleaning effect on wafers. At the same time, some chemicals used in semiconductor and liquid crystal factories will increasingly tend to contain a surfactant in line with the progress of the microfabrication. As a typical example of such chemical may be mentioned a surfactant-containing buffered hydrofluoric acid.
Hitherto, for treatment of waste water containing fluorine and surfactants discharged from various types of industrial establishments and also from semiconductor and liquid crystal factories, there has been known a method wherein a large amount of slaked lime is added to the waste water relative to the fluorine contained therein to produce a hardly soluble calcium fluoride so that the fluorine is precipitated for chemicophysical removal. After the step of fluorine treatment, organic matter, such as surfactant, is microbiologically (biologically) treated in a tank other than for fluorine treatment while nutrients, such as phosphoric acid and urea, being added to the tank.
In semiconductor and/or liquid crystal factories having no facility for microbial waste water treatment, it has been usual practice to collect organic substances, such as surfactants, as much as possible at the manufacturing process and store the collected organic substances in a tank installed at a separate location, which substances are in turn transferred to an independent firm for treatment.
Conventional apparatuses for treatment of waste water containing fluorine and surfactants include, for example, one illustrated in FIG. 8 or FIG. 9. FIG. 8 shows a two-stage coagulating sedimentation system. FIG. 9 shows a two-stage coagulating sedimentation system including a microbial treatment stage. The FIG. 8 system is identical with the FIG. 9 system except that a biological contact oxidation tank 28 is not included therein.
The conventional arrangement shown in FIG. 8 will be explained. In the waste water treating apparatus shown, waste water containing fluorine and surfactants is initially caused to flow into a raw waste water tank 14. In the raw waste water tank 14, the fluorine-containing waste water is subjected to some degree of adjustment in both quantity and quality. Thereafter, the waste water is transferred by a waste water tank pump 9 into a first reaction tank 15. In the first reaction tank 15, the fluorine-containing water is loaded with slaked lime and agitated by an agitator 12 for reaction so that fluorine dissolved in the waste water is caused to react with the slaked lime into calcium fluoride. Where the time for reaction of fluorine with the calcium in the slaked lime is relatively short, an appreciable amount of slaked lime is added. Where the pH of the fluorine-containing waste water is considerably low, an adequate amount of slaked lime is added for neutralization.
Then, the waste water is transferred to a second reaction tank 16 in which an aluminum agent is added to the waste water for reaction with unreacted fluorine to produce aluminum fluoride. Fine flocs of calcium fluoride and aluminum fluoride are wrapped with aluminum hydroxide produced by aluminum agent additions.
Nextly, in a first coagulation tank 17, a polymer coagulant is added to the waste water for reaction so that fine flocs of calcium fluoride produced by the reaction in the first reaction tank 15 and fine flocs of aluminum fluoride produced by the reaction in the second reaction tank 16 are grown into larger flocs. Then, the waste water is subjected to solid-liquid separation in a first settling tank 18. A larger part of the slurry including flocs subjected to solid-liquid separation is a hydroxide resulting from excess slaked lime. Although the slurry contains reaction products of calcium fluoride and aluminum fluoride, such reaction products are small in quantity and a larger part of them is a sludge attributable to unreacted chemicals.
In the foregoing stages, the fluorine concentration of the waste water cannot be reduced to the level of a target water quality or not more than 15 ppm. Therefore, the waste water is transferred sequentially to a third reaction tank 19, a second coagulation tank 20, and a second settling tank 21 to reduce the fluorine concentration to the target level. Possibly, the pH in the second settling tank 21 may not come within the effluent standard because of the use of an acid chemical in the third reaction tank 19 or because of the reaction time in the third reaction tank 19 being so short. Therefore, the waste water from the second settling tank 21 is then passed through a pH adjust tank 22 so that the pH of the waste water falls within the range of the effluent quality standard. Thereafter, the waste water is discharged.
Masses of slurry collected from the first settling tank 18 and second settling tank 21 by a collector 13 are concentrated in a sludge concentration tank 5 and dewatered by a dehydrator 6 into a cake having a prescribed water content.
The other conventional arrangement shown in FIG. 9 further includes a microbial treatment stage between the first and second coagulating sedimentation stages. The waste water is introduced from the first settling tank 18 into a contact oxidation tank 28 in which fillers 29 are packed, and surfactants present in the waste water are treated by aerobic microorganisms present in the tank 28. The fillers 29 are corrugated filter mediums made of vinyl chloride or plastics.
However, this contact oxidation tank 28 has involved a problem that even when nutriments or the like which provide good culture conditions for microorganisms are added to the tank, surfactants cannot easily be microbially degraded because of the molecular and structural formulas, germicidal effect, and foamability of the surfactants per se. As such, the trouble is that the treated water remains foamable, and therefore, it has been necessary to add an antifoaming agent or the like anew in order to eliminate such foamability of treated water.
In semiconductor factories, the fluorine concentration of fluorine-containing waste water is always subject to wide variations within a range of 30 to 300 ppm. In actuality, therefore, the quantity of slaked lime required against fluorine in a fluorine/slaked lime reaction is more than three times a stoichiometric quantity for such reaction.
Generally, in semiconductor factories, the pH of fluorine-containing waste water is low and often within the range of about 2 to 3, because the waste water comes from the stage of wafer fabrication in which acid is used.
Therefore, waste water produced in such factories contains fluorine, is rather low in pH value, and is likely to vary in quality. This requires use of slaked lime and the like in large quantities for fluorine treatment and neutralization, which means that large quantities of wastes are finally discharged in the form of cake and otherwise. This fact poses not only the problem of disposal cost, but also great concern over the possibility of securing sites for future waste disposal. That is, a large quantity of cake is generated as a result of sludge being dewatered by a dehydrator 6, and large quantities of such cake, as industrial wastes, are disposed of by landfilling. In other words, semiconductor and liquid crystal factories generate large quantities of waste water which contain fluorine and organic substances, such as surfactants, and this necessitates various considerations including the provision of facilities for treatment of such waste water, keeping the quality of treated waste water up to the effluent standard, and measures for disposal of the increasing resulting wastes. Indeed, industrial wastes generated in such factories have continued to steadily increase. Thus, urgent necessity exists for proper measures against ever growing problems of wastes, including the problem of landfill site availability.
The above noted problems will be further discussed in detail. The conventional method of treating fluorine by addition of slaked lime or the like has a disadvantage that since the pH of fluorine-containing waste water is generally low and since the fluorine concentration of fluorine-containing waste water is subject to wide variation, it is impracticable to properly control the quantity of slaked lime required for neutralization and reaction control. This virtually leads to excessive use of slaked lime. Generally, in semiconductor factories, slaked lime of more than three times as much as a stoichiometric quantity has been used as earlier noted. Addition of more than three times as much slaked lime as the theoretical requirement naturally results in an increase in the proportion of unreacted calcium, which in turn leads to increased generation of .hydroxide or the like sludge. After dewatered by a dehydrator, such as filter press, such sludge still has a water content of the order of 65%, which means a large volume of cake. Another reason why actual slaked lime requirement is more than three times the theoretical requirement is that the time for reaction of fluorine-containing waste water with slaked lime added is generally set at not more than 30 minutes, which necessitates excessive loading of slaked lime in order to achieve a sufficient degree of fluorine removal; otherwise, a target value at the first stage of coagulating sedimentation (generally 20 to 30 ppm) cannot be achieved. Conversely, if sufficient time is taken for reaction, it is necessary that the volume of the reaction tank be made more than three times as large as the existing one, which means a size greater than the agitator. This is unreasonable and uneconomical judging from site availability and construction cost.
Treatment of surfactants involves a technical problem arising especially from the fact that surfactants have a germicidal effect upon microorganisms, which makes it difficult to microbially treat the surfactants.
Administrative regulation on the concentration of fluorine in treated waste water is becoming increasingly severe year after year, and currently industrial establishments have their regulation values for fluorine established in consideration of the current administrative standards, which values, in many cases, are of a single digit on the order of several ppm. However, in order to maintain the fluorine concentration in treated waste water at a single digit level, insofar as existing facilities are employed, it is impracticable to achieve a target fluorine concentration unless more than ten times as much aluminum agent as the fluorine concentration is added. With a water treating apparatus having no contact oxidation tank 28 as shown in FIG. 8, waste water containing organic substances, such as surfactants, cannot be microbially treated. This involves the problem of effluent quality deterioration, particularly in COD (Chemical Oxygen Demand).
Today, reasonable and economical treatment of waste water, and weight reduction of wastes are strongly called for as stringent public needs. This is of vital importance from the standpoint of global environment protection as well. In particular, positive maintenance to a reasonable quality level of treated waste water from semiconductor and liquid crystal factories, as well as weight reduction of wastes, is an urgent task required to be solved.
Hitherto, fluorine-containing waste water and hydrogen peroxide-containing waste water have been separately treated for discharge. The reason for this is that presence of hydrogen peroxide in fluorine-containing waste water adversely affects the process of coagulating sedimentation, and that hydrogen peroxide per se acts as a COD source, thus increasing the COD value of the effluent. Hydrogen peroxide-containing waste water has been treated by adding a hydrogen peroxide decomposing chemical (e.g., KURIBARTER, a trade name) thereto. Waste water containing hydrogen peroxide in which the concentration of hydrogen peroxide is not more than 1000 ppm is first introduced into a hydrogen peroxide-containing waste water tank wherein the waste water is subjected to some adjustment in volume and quality, and is then transferred by a pump into a hydrogen peroxide decomposition column which is loaded with activated carbon as a catalyst. FIG. 10 shows such a conventional system, wherein the hydrogen peroxide-containing waste water tank is indicated by numeral 55, the pump is indicated by numeral 59 and the hydrogen peroxide decomposition tower is indicated by numeral 56. In the hydrogen peroxide decomposition tank 56, is the hydrogen peroxide-containing waste water catalytically decomposed into water and oxygen gas, with the activated carbon as a catalyst, and after decomposition, separated components are introduced into a hydrogen peroxide treating tank 57 for further treatment.
However, for purposes of piping arrangement at production stages which discharge fluorine and hydrogen peroxide, it has been extremely difficult to arrange for positive prevention of hydrogen peroxide from inclusion into fluorine-containing waste water, because of many production units involved and constructional complexity of the production equipment itself. As such, the development of a waste water treating apparatus which can treat waste water containing hydrogen peroxide and fluorine in one complete arrangement has been eagerly desired.
In each prior art arrangement shown, the raw waste water tank 14 is a tank operative to perform volume and quality adjustment only with respect to waste water. This tank is volumewise large-sized only by reason of retention time, but has no reaction function relative to fluorine. So, the tank is not meaningful in terms of investment efficiency. Therefore, it has been desired that a raw waste water tank be provided which is effectively designed for waste water treatment to enhance investment efficiency.
In semiconductor and liquid crystal factories, there exist domestic waste water treatment facilities for treating domestic waste water from a dining hall, bathrooms, lavatories, and so on. Such domestic waste water treatment facilities generates highly nutrient domestic excess sludge, but in such factories no particular way is found for effective utilization of such domestic excess sludge. In practice, such excess sludge has been disposed of by an independent firm at the expense of each factory.